1. Field of the Invention
This invention relates generally to vacuum sputter coating apparatus, and more particularly to the non-mechanical attachment of a sputter target to the cathode of a planar magnetron sputtering coating source.
2. Description of Related Art
Cathodic sputtering refers to the erosion of the cathode by ion bombardment that occurs when an electrical discharge is passed between electrodes in a low pressure gas. In the sputtering process inert gas ions with a positive charge are accelerated from the glow discharge, that forms between the electrodes, to the negative cathode. Erosion results from the ejection of atoms and clusters of atoms from the cathode surface as a result of momentum transfer from the bombarding ions. Some of the ejected cathode material condenses on surfaces surrounding the cathode. Sputtering becomes a coating process when the ejected material is deliberately condensed on a substrate suitably positioned near the cathode.
Sputtering is a vacuum coating process where an electrically isolated cathode is mounted in a chamber that can be evacuated and partially filled with an inert gas. If the cathode material is an electrical conductor, a direct-current high-voltage power supply is used to apply the high voltage potential. If the cathode is an electrical insulator, the polarity of the electrodes must be reversed at very high frequencies to prevent the formation of a positive charge on the cathode that would stop the ion bombardment process. Since the electrode polarity is reversed at a radio frequency of 13.56 MHz, this process is referred to as RF-sputtering.
Magnetron sputtering is a more efficient form of diode sputtering that uses a magnetic field to trap electrons in a region near the target surface creating a higher probability of ionizing a gas atom. The high density of ions created near the target surface causes material to be removed many times faster than in diode sputtering. The magnetron effect is created by an array of permanent magnets included within the cathode assembly that produce a magnetic field normal to the electric field.
Ion bombardment not only causes atoms of the target material to be ejected, but it also imparts considerable thermal energy to the target. Consequently, any target attachment scheme must provide for good physical contact to the cathode assembly to allow adequate thermal transfer of the target's heat to the cooling media. This is particularly true in the case of magnetron sputtering where very large ion currents are produced causing a very intense and localized heating of the target.
Various means have been used in the past for holding sputter targets in place within the sputter sources. Commercially available sputter targets today are either bonded directly to the cathode assembly or secured using various mechanical means. The method used to attach the sputter target to the cathode assembly will also greatly affect the size and overall design of the magnetron source, the amount of down time when changing targets, and the overall performance of the source.
In some sputter sources, the cathode targets are positioned adjacent to a fixed backing plate and retained in place by various holding devices. U.S. Pat. Nos. 5,021,139, 4,915,805, and 4,421,628 teach the use of screws to tightly clamp the target to the backing plate. U.S. Pat. No. 4,417,968 shows the use of special end brackets to hold novel cylindrical targets. These brackets are held in place by screws.
U.S. Pat. Nos. 4,812,217 and 4,761,218 disclose the use of mechanically fastened clamps to retain the target against the backing plate. Because these designs have clamping assemblies in line with the cathode, the use of sputter shields over the clamping assembly is required to protect the substrate from being contaminated with undesirable material ejected from the clamping assembly. U.S. Pat. No. 4,049,533 teaches the use of an elastic metal clamping element to retain the cathode in a cooling assembly. Because the elastic clamps are not in line with the cathode, the need for contamination shielding is not required.
In the above designs, the sputtering target is independently removable from the fixed backing plate and cooling assembly for changing or replacing the targets.
In other versions, the sputtering target is physically soldered to a backing plate, usually where the backing plate and the target are inserted and removed as a unit. The advantages of this over mechanical target attachment are firmly retaining the target in position and establishing more intimate thermal contact between the target and the backing plate/cooling assembly.
U.S. Pat. Nos. 5,009,765 and 4,183,797 show the use of joined target-backing plate assemblies. The former uses a complementary holding fixture for securing the welded assembly and the latter suggests that the joined assembly is an integral part of the total sputtering apparatus. U.S. Pat. No. 5,022,978 also suggests the use of a joined target-backing plate assembly as shown in FIG. 2.
In all of the joined target-backing plate designs, when the target is replaced, either both the spent target and the corresponding backing plate must be discarded or a long and tedious target removal operation is required. Also, during target removal from the backing plate it is easy to damage the target backing plate. Further, in designs where the joined assembly is an integral part of the complete system, the difficulty in changing or replacing targets becomes overly burdensome.
Other novel attachment schemes have also been disclosed. U.S. Pat. No. 4,392,939 teaches the use of a vacuum connection where the backing plate has a number of channels which about the contact face of the target and allow the target to be secured against the backing plate by vacuum suction via the channels. This concept, although unique, involves costly precision machining of the vacuum interfaces to be mated and the use of additional plumbing and vacuum systems. A possible advantage of a vacuum attachment over mechanical attachments is that it offers a more uniform thermal contact between the target and backing plate, without all the complexities associated with bonded assemblies.
U.S. Pat. No. 4,204,936 discloses the use of a ferromagnetic target retainer which releasably clamps the target to the backing plate by virtue of its attraction to existing magnetic fields in the cathode assembly. The retainer engages a lip in the central aperture of the sputter target allowing it to press the back surface of the target against the cathode assembly. In all the designs, the ferromagnetic retainer is exposed in the center and at the surface of the sputter target. Although the retainer may be located outside the erosion area of the sputter target, it is a very real source of contamination in the sputtering process. All target attachment methods that employ clamping assemblies in line with the cathode use sputter shields to prevent sputtering of the clamping assembly as described in U.S. Pat. Nos. 4,812,217 and 4,761,218. An additional disadvantage of these designs is the central aperture in the sputter target that will not allow the retrofit to existing planar magnetron sputter sources that utilize a solid target.